[PATCH] vc_screen: break from vcs_read() while loop if vcs_vc() returns NULL

[George Kennedy]

If vcs_vc() returns NULL in vcs_read(), break if partial read,
else if no reads have been done, go to unlock_out and return ENXIO.

Fixes: 226fae124b2d ("vc_screen: move load of struct vc_data pointer in vcs_read() to avoid UAF")
Reported-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: George Kennedy <george.kennedy@xxxxxxxxxx>
---
 drivers/tty/vt/vc_screen.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/vt/vc_screen.c b/drivers/tty/vt/vc_screen.c
index f566eb1839dc..29288401cf9e 100644
--- a/drivers/tty/vt/vc_screen.c
+++ b/drivers/tty/vt/vc_screen.c
@@ -403,10 +403,13 @@ vcs_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
 		unsigned int this_round, skip = 0;
 		int size;
 
-		ret = -ENXIO;
 		vc = vcs_vc(inode, &viewed);
-		if (!vc)
+		if (!vc) {
+			if (read)
+				break;
+			ret = -ENXIO;
 			goto unlock_out;
+		}
 
 		/* Check whether we are above size each round,
 		 * as copy_to_user at the end of this loop
-- 
2.31.1